Negative impedance converter

ABSTRACT

There is provided a negative impedance converter, which has negative impedance conversion function by widening the available range of a generalized impedance converter.  
     A generalized impedance converter is composed of two operational amplifiers Q 1  and Q 2  and four series-connected first to fifth impedance elements Z 1  to Z 4 . The four impedance elements included in the generalized impedance converter are all set as the same resistor R 1 , and an impedance element Z 6  is connected between the intermediate point B of the series-connected impedance elements and the ground. The magnitude of the impedance element Z 6  is set smaller than that of load impedance element Z 5 . By doing so, the input impedance Z 11′  becomes negative, and the kind of the impedance is determined depending on the kind of the impedance elements Z 5  and Z 6 .

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a negative impedance converter, which can make a desired kind of negative impedance conversion by widening the available range of a generalized impedance converter.

[0003] 2. Description of the Related Art

[0004] An impedance converter makes conversion between different impedances in addition to the same impedances. A generalized impedance converter is given as the conventional impedance converter. (For example, see A. Antoniou, “Realization of Gyrators Using Operational Amplifiers, and Their Use in RC Active Network Synthesis”, Proc. IEE, vol. 116, pp. 1838-1850, 1969) The generalized impedance converter comprises a circuit ZIC combining two operational amplifiers Q₁, Q₂ and four impedance elements Z₁ to Z₄, as seen from the dashed line frame in FIG. 6. The input impedance Z_(11′) to convert the negative impedance Z₅ is expressed by the following equation (1). $\begin{matrix} {Z_{11} = {\frac{Z_{1}Z_{3}}{Z_{2}Z_{4}}Z_{5}}} & (1) \end{matrix}$

[0005] More specifically, various impedance elements are used as four impedance elements Z₁ to Z₄. By doing so, the foregoing generalized impedance converter ZIC can make various impedances proportional to the magnitude of the negative impedance Z₅. For example, when the impedance element Z₄ is set as capacitor, the impedance is 1/sC₄. Further, when impedance elements Z₁ to Z₃ are individually set as the same resistor R1, and the negative impedance Z₅ is set as resistor R₂, the input impedance Z_(11′) is expressed by the following equation (2).

Z _(11′) =sR ₁ R ₂ C ₄  (2)

[0006] Therefore, inductor is equivalently obtained.

[0007] In addition, when impedance elements Z₁ and Z₅ are set as capacitors C₁ and C₅, respectively, the impedance is 1/sC₁, 1/Cs₅; in this case, the input impedance Z_(11′) is expressed by the following equation (3). $\begin{matrix} {Z_{11^{\prime}} = \frac{1}{s^{2}R_{1}C_{1}C_{5}}} & (3) \end{matrix}$

[0008] If s=jω (ω=2πf), the following equation (4) is obtained. $\begin{matrix} {{Z_{11^{\prime}}\left( {j\quad \omega} \right)} = \frac{- 1}{R_{1}C_{1}C_{5}\omega^{2}}} & (4) \end{matrix}$

[0009] The polarity of the input impedance Z_(11′) is negative; therefore, a negative resistor called frequency-dependent negative resistor (FDNR) is obtained. In this case, the frequency-dependent negative resistor (FDNR) has property inversely proportional to ω², that is, frequency f. Thus, the frequency-dependent negative resistor (FDNR) is applicable as a new ladder-type filter.

[0010] As described above, the conventional generalized impedance converter can make mutual conversion between different impedance as capacitor and inductor, and negative resistor inversely proportional to ω².

[0011] However, the conventional generalized impedance converter cannot make negative capacitor and negative inductor.

[0012] The present invention has been made in view of the foregoing circumstances. It is, therefore, an object of the present invention to provide a negative impedance converter, which readily can obtain negative capacitor and negative inductor in addition to negative resistor by widening the available range using a generalized impedance converter circuit.

SUMMARY OF THE INVENTION

[0013] According to one aspect of the present invention, there is provided a negative impedance converter comprising: four series-connected first to fifth impedance elements; and two operational amplifier circuits having input and output connected to form negative feedback loop with respect to the first to fourth impedance elements, a sixth impedance element being connected between the central connection point of the first to fourth impedance elements and two operational amplifier circuits and the ground, a value of the sixth impedance element being set smaller than a value of a fifth impedance element so that the input impedance becomes negative, the fifth impedance element functioning as a load connected to the termination of the first to fourth impedance elements.

[0014] The negative impedance converter of the present invention has the following configurations.

[0015] (1) The first to sixth impedance elements are all resistors and the value of the first to fourth resistance values are set to the same, and thereby, the input impedance becomes negative resistance.

[0016] (2) The first to fourth impedance elements are all resistor having the same resistance value and the fifth and sixth impedance elements are set as inductor, and thereby, the input impedance becomes negative inductance.

[0017] (3) The first to fourth impedance elements are all resistor having the same resistance value and the fifth and sixth impedance elements are set as capacitor, and thereby, the input impedance becomes negative capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a circuit diagram showing the basic configuration of a negative impedance converter of the present invention;

[0019]FIG. 2 is a circuit diagram to explain the operation principles of a negative impedance converter of the present invention;

[0020]FIG. 3 is a circuit diagram showing the configuration of a negative impedance converter according to one embodiment of the present invention;

[0021]FIG. 4 is a circuit diagram showing the configuration of a negative impedance converter according to another embodiment of the present invention;

[0022]FIG. 5 is a circuit diagram showing the configuration of a negative impedance converter according to another embodiment of the present invention; and

[0023]FIG. 6 is a circuit diagram showing the configuration of a conventional generalized impedance converter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024]FIG. 1 shows the basic circuit configuration of a negative impedance converter of the present invention. In FIG. 1, Z₁ to Z₄ denote first to fourth impedance elements, respectively, and Z₅ denotes a fifth impedance element functioning as a load. Q₁ and Q₂ denote operational amplifier circuits. Four, that is, first to fourth impedance elements Z₁ to Z₄ are connected in series, and two operational amplifier circuits Q₁ and Q₂ are connected to form negative feedback loops n₁ and n₂ with respect to these impedance elements Z₁ to Z₄ Z₆ is a sixth impedance element, which is connected between the point B and ground.

[0025] Three points connected with input terminals of two operational amplifier circuits Q₁ and Q₂ are set as A, B and C with respect to these impedance elements Z₁ to Z₄. By doing so, three points as A, B and C operate at the approximately same potential because two operational amplifier circuits Q₁ and Q₂ are connected to form negative feedback loops n₁ and n₂ as described above. Thus, these three points are regarded as being virtually connected. As illustrated in FIG. 1, the sixth impedance element Z₆ is connected between the point B and the ground, and the value is set as Z₅>Z₆. By doing so, current flowing through the impedance element Z₆ is not only supplied via the impedance element Z₅, but also sucked from an external circuit (not shown) connected to input terminals 1 and 1′. Therefore, the input impedance Z_(11′) has negative polarity.

[0026] In the following description, the operation principles will be explained in detail with reference to FIG. 2. In FIG. 2, impedance elements Z₁ to Z₄ are all set as the same resistor R₁.

[0027] In the circuit shown in FIG. 2, the following four equations (5) to (8) are given. $\begin{matrix} {\frac{1}{I_{1}} = \frac{R_{1}}{V_{1} - V_{2}}} & (5) \end{matrix}$

V₁=V₃=V₅  (6) $\begin{matrix} {V_{3} = {\left( {V_{2} + V_{4}} \right)\frac{Z_{6}}{R_{1} + {2Z_{6}}}}} & (7) \\ {V_{5} = {V_{4}\frac{Z_{5}}{R_{1} + Z_{5}}}} & (8) \end{matrix}$

[0028] From the foregoing four equations (5) to (8), input impedance Z_(11′), that is, f₁/I₁ can be found. Thus, the input impedance Z_(11′) is expressed by the following equation (9). $\begin{matrix} {Z_{11^{\prime}} = {\frac{V_{1}}{I_{1}} = \frac{Z_{5}Z_{6}}{Z_{6} - Z_{5}}}} & (9) \end{matrix}$

[0029] Therefore, the input impedance Z_(11′) is determined by only Z₅ and Z₆.

[0030] From the foregoing explanation, the following matters 1 to 3 are given.

[0031] 1: When Z₅=R₅, Z₆=R₆, ${Z_{11^{\prime}} = \frac{R_{5}R_{6}}{R_{6} - R_{5}}};$

[0032]  therefore, when R₅>R₆, negative resistor is obtained.

[0033] 2: When Z₅=sL₅, Z₆=sL₆, ${Z_{11^{\prime}} = \frac{{sL}_{5}L_{6}}{L_{6} - L_{5}}};$

[0034]  therefore, when L₅>L₆, negative inductor is obtained.

[0035] 3: When ${Z_{5} = \frac{1}{{sC}_{5}}},{Z_{6} = \frac{1}{{sC}_{6}}},{{Z_{11^{\prime}} = {\frac{\frac{1}{s^{2}C_{5}C_{6}}}{\frac{1}{{sC}_{6}} - \frac{1}{{sC}_{5}}} = \frac{1}{s\left( {C_{5} - C_{6}} \right)}}};}$

[0036] therefore, when C₆>C₅, negative capacitor is obtained.

[0037]FIG. 3 to FIG. 5 shows each embodiment of the present invention.

[0038]FIG. 3 shows a circuit for obtaining negative resistance as the input impedance Z_(11′). As described above, R₅>R₆ is set, and thereby, negative resistance of input impedance (input resistance) Z_(11′)=−R₅R₆/(R₅−R₆).

[0039]FIG. 4 shows a circuit for obtaining negative inductance as the input inductance. As described above, L₅>L₆ is set, and thereby, input impedance Z_(11′)=s²L₅L₆/(L₆−L₅); therefore, negative inductance of −L₅L₆/(L₆−L₅) is obtained as the input inductance.

[0040]FIG. 5 shows a circuit for obtaining negative capacitance as the input capacitance. As described above, C₆>C₅ is set, and thereby, input impedance Z_(11′)=1/s(C₅−C₆); therefore, negative capacitance of −(C₆−C₅) is obtained as the input capacitance.

[0041] As seen from the foregoing explanation, it is possible to convert positive resistance, inductor and capacitor into negative resistance, inductor and capacitor using the generalized impedance converter.

[0042] The generalized impedance converter of FIG. 6 used as the base of the present invention has the following configuration. More specifically, two terminations (i.e., points A and B in FIG. 1) of four serial-connected impedance elements Z₁ to Z₄ are connected to (+) input terminals of two operational amplifiers Q₁ and Q₂ In addition, the central connection point (i.e., point B in FIG. 1) of the impedance elements Z₁ to Z₄ is connected to (−) input terminals of two operational amplifiers Q₁ and Q₂.

[0043] The present invention presumes that the circuit configuration described above is used. However, the present invention has no limited requirement as to whether any of two (+) and (−) input terminals of two operational amplifiers Q_(1 and Q) ₂ should be connected to the terminations or the central connection point. The reason is because the conclusion has no change even if the connection polarity of the input terminal is plus or minus so long as two operational amplifiers Q₁ and Q₂ has open loop gain in some degree and the foregoing equation (6) is established.

[0044] As is evident from the foregoing description, according to the present invention, it is possible to make the following conversion, which has not been made so far by the generalized impedance converter. One is conversion from positive resistor into negative resistor. Another is conversion from positive inductor into negative inductor. Another is conversion from positive capacitor into negative capacitor. Therefore, it is possible to largely widen the available range of the generalized impedance converter. 

What is claimed is:
 1. A negative impedance converter comprising: four series-connected first to fifth impedance elements; and two operational amplifier circuits having input and output connected to form negative feedback loop with respect to the first to fourth impedance elements, a sixth impedance element being connected between the central connection point of the first to fourth impedance elements and two operational amplifier circuits and the ground, a value of the sixth impedance element being set smaller than a value of a fifth impedance element so that the input impedance becomes negative, the fifth impedance element functioning as a load connected to the termination of the first to fourth impedance elements.
 2. The negative impedance converter according to claim 1, wherein the first to sixth impedance elements are all resistor, and the value of the first to fourth resistance values are set to the same so that the input impedance becomes negative resistance.
 3. The negative impedance converter according to claim 1, wherein the first to fourth impedance elements are all resistor having the same resistance value, and the fifth and sixth impedance elements are set as inductor so that the input impedance becomes negative inductance.
 4. The negative impedance converter according to claim 1, wherein the first to fourth impedance elements are all resistor having the same resistance value, and the fifth and sixth impedance elements are set as capacitor so that the input impedance becomes negative capacitance. 